Physiological mechanisms underlying photoperiodic regulation of larval growth and larval diapause will be investigated. Characteristics of larval growth rate responses to different photoperiodic regimes will be determined for the black cutworm, Agrotis ipsilon. Effects of photoperiod on the physiology of larval growth will be studied in terms of feeding, assimilation, metabolism, and hormonal control of the molting cycle. The role of photoperiod in inducing and mantaining larval diapause will be studied in the Europea corn borer, Ostrinia nubilalis. This aspect of photoperiodism will be studied in terms of the production and metabolism of juvenile hormone (JH), ecdysone, and the functions of the neurosecretory system. Particular emphasis will be placed on the role of the lateral neurosecretory cells of the protocerebrum in biological clock functions; this study will involve ultrastructural examination of secretory processes and membrane configurations that may exhibit circadian time-related sequential changes. The relationship of JH to diapause induction will be studied by means of determining temporal profiles of hemolymph JH titers, JH esterase, JH binding proteins; these determinations will be under long-day (nondiapause) and short-day (diapause-inducing) photoperiods. Effects of exogenous factors (JH mimics, biological clock inhibitors) on the profiles will be determined. Ecdysone activity profiles will also be determined under the above experimental conditions, using radioimmunoassay methods.